A long-legged white crane flits languidly through a paddy field as birdsong fills the air among the lush green hills. Welcome to possibly the most dangerous nature haven on the planet – the demilitarised zone separating South and North Korea.

But the biggest dangers are not the tigers reputed to roam there, or even the bears and leopards, but the millions of land mines and tank traps. In addition to the weaponry, North Korea's 1 million-man army faces the south's 600,000 troops, backed by 28,000 US ground troops, each side glaring theatrically at each other across the military demarcation line in Panmunjom at the 38th parallel.

Nonetheless, whilst political stability holds, foreign tourists – not Koreans - are being allowed to visit a small safe part of the zone, in Panmunjon, on bus trips from Seoul (there are no plans to clear the area of mines but estimates run to $1bn to do the job). The atmosphere is more Disneyland with a military twist than a potential flashpoint for a third world war, a mood reinforced by sales of rusty barbed wire in the tourist shop.

Environmentally, the demilitarized zone (DMZ) is unique. Nowhere else in the world has a patch of land that was worked by farmers for over 5,000 years suddenly been allowed to revert to a true state of nature. Ever since the 1953 armistice was signed - although not by the two Koreas - the strip of land, 2.4 miles wide and 155 miles long, has developed into a de facto nature reserve, untouched by human activity.

For the past five decades, the battlefields of the DMZ, on which 4 million were killed or wounded, have returned to thick prairie and shrub in the west and rich green forests in the eastern mountain ranges.

Endangered plants and animals have thrived. The Asiatic black bear, leopard, Eurasian lynx, and Goral sheep have made a comeback, and perhaps even the very rare Amur tiger - which some US soldiers say has been captured on video by surveillance teams. Many migratory birds, including the white-naped and red-crowned cranes and the black-faced spoonbill have made their homes in the DMZ. Environmentalists say over 1,100 plant species and mammal species exist in this wildlife sanctuary.

But environmentalists worry over how long the DMZ will remain in its pristine state. Development is creeping right up to the barbed wire in the south, now the world's 13th largest economy. Deforestation has caused severe flooding to the north of the DMZ.

The Gaeseong industrial park, which opened in 2004, could be a sign of things to come. Visible in the distance from Panmunjom, the industrial zone combines cheap North Korean labour with South Korean investment as part of Seoul's attempt to woo Pyongyang out of its isolation. Six miles north of the DMZ, Gaeseong has direct road and rail access to South Korea and is just one hour's drive from Seoul. Should Korea ever become united – and remember the speed at which Germany united - the pressure to develop the DMZ could be irresistible, calling time on this wilderness.

Mindful of this risk, Ke Chung Kim, 74, founded the DMZ Forum in 1994 in what must be one of the world's most ambitious swords into ploughshare projects - preserving the DMZ as a nature reserve under the auspices of a Unesco world heritage site. Kim has won the backing of Nelson Mandela, Kofi Annan and others, and annual conferences have been held since 2003.

A professor of entomology, Kim is curator and director at the centre for biodiversity research at Pennsylvania state university. He will attend a wetlands conference in Seoul in October when he will again lobby the South Korean government to try and persuade Pyongyang to come to an agreement on preserving the DMZ as a nature reserve.

"I feel we're at a make or break point," he told the Guardian, as the developers get ever nearer to the DMZ.

But if Kim is having trouble getting his idea on the national political agenda in the south, the challenges are even more forbidding with the north. "North Korean scientists and intellectuals see the value of a nature reserve, but as long as the military control the government, it won't be top of their agenda," Kim lamented.

Kim's job is all the tougher as relations between the two Koreas have been frozen since the North Korean capital, Pyongyang, reacted badly to the more robust approach to diplomacy adopted by the South Korea's new president, Lee Myung-bak, who came into office in February.

However, there is support for Kim from local officials. Kim Moon-soo, the governor of Geyonggi-do, one of the two provinces in South Korea abutting the DMZ, has raised the intriguing idea of a north-south bike path as well as a small eco-park.

"They would be confidence-building measures, if you like, to try and regain some momentum after the recent deterioration in relations," said T H Lee, spokesman for governor Kim.

Lee, who spent one night in the DMZ in the 1980s as part of his military service training, is bullish about plans to turn the DMZ into a nature reserve: "The DMZ was once a symbol of conflict, it should now be turned into a symbol of peace."

Lee, who is also a successful investor, takes a hard line against any sort of development within the DMZ, including hotels, no matter how tempting the opportunity for a unique resort. He insisted that there was no need for accommodation that would despoil the area's natural beauty. "You can stay in Seoul, the one-hour drive is part of the experience."

Lee is right because the sights and memorials on the way – including one to the 18 killed war correspondents – is part of the build-up to the DMZ. Apart from being a de-facto nature reserve, the DMZ is considered sacred ground, with the remains of tens of thousands of casualties of war still undiscovered.

The challenges of preserving the DMZ as a unique nature reserve are formidable, starting with North Korean leader Kim Jong-il. It is likely his priority would be rapid development as a means of bringing in badly-needed foreign exchange. Moreover, the DMZ's future lies not just with the north and south, but with the US, China and Russia, the signatories to the 1953 armistice.

Yet notwithstanding all the obstacles, Professor Kim clings steadfastly to his vision of the DMZ.

"Without biodiversity, Korea's future will be short-changed so we are determined to protect the DMZ and preserve it permanently," he said. "Many of the species that disappeared from the rest of Korea are still there. There is a lot at stake here."